Broadcast system for a control signal with interference reduction

ABSTRACT

A system for controlling, from the sending side, the receiving side in a broadcast system using a control signal, where a false signal similar to the regular control signal is detected from the broadcast signals before the receiving side is spuriously triggered by the false signal. Elimination filter or filters is/are inserted in the path of the broadcast signals during only a predetermined time to eliminate the same frequency components as the control signal, whereby the control signal can be transmitted with no chance of spurious triggering.

United States Patent Fukata Feb. 8, 1972 [54] BROADCAST SYSTEM FOR A 3,391,340 7/1968 Fyler et al ..325/64 CONTROL SIGNAL WITH I INTERFERENCE REDUCTION EmmwRbm L-Rwhard$n Attorney-Robert M. Dunning [72] Inventor: Masayuki Fukata, 94, Shimorenjaku,

Metaka-shi, Tokyo, Japan [57] ABSTRACT [22] Filed: Dec. 16, 1969 A system for controlling, from the sending side, the receiving side in a broadcast system using a control signal, where a false [211 App! signal similar to the regular control signal is detected from the broadcast signals before the receiving side is spuriously trig- [52] US. Cl. ..325/64, 325/65, 325/364 gered by the false signal. Elimination filter or filters is/are in- [51] Int. Cl. "04b 1/06 serted in the path of the broadcast signals during only a [58] Field of Search "325/55, 64, 65, 364; 343/225, predetermined time to eliminate the same frequency com- 343/228 ponents as the control signal, whereby the control signal can be transmitted with no chance of spurious triggering. [56] References Cited 4 Claims, 10 Drawing Figures UNITED STATES PATENTS 3,061,783 10/1962 Noller V 6 RE(C A? SP T/ I I RECEIVER 5 l AMPLIFIER l I F i r CONTROL I Q A l I CIRCUIT J I I PATENTEU '8 I973 SHEET 2 BF 6 T m w A L U D nu M n C S U r IL R m 0 M A 5 m M 6m fl .I 0 5 .9 a 8 'll 1 0 w F. l L 7 R R k H am 0 N R w I 0 SU T Y um m M mu m a Y P M MOD M A r T U m m mm M, Sc LT W m? 4/ H RM Mm (/U 00. w H T Wm M ,w

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INVENTOR.

Masagukl Fukafa TAASMQM ATIY.

BROADCAST SYSTEM FOR A CONTROL SIGNAL WITI'I INTERFERENCE REDUCTION This invention relates to a system for controlling, from the sending side, the receiving side in a broadcast system, and more particularly to (1) an emergency broadcast system for transmitting emergency information, such as information of a natural disaster etc., from the broadcast station to the receivers, (2) a recorder-control broadcast system for causing recorders of the receivers, such as tape recorders, to record the broadcast information in accordance with the control of the broadcast station, or (3) a broadcast system for a control signal transmitted to control the switch operation of each of various kinds of apparatus at the receiving side.

In the conventional system of the emergency broadcast system type, the emergency control signal is sent out from the sending side, such as the broadcast station, before the broadcast of an emergency information. If a broadcast program is being sent out, the emergency control signal is sent out together with the broadcast program. However, if the broadcast program is interrupted, only the emergency control signal is sent out. The emergency control signal is generally a continuous or intermittent wave or waves each having a predetermined frequency, or it may be a frequency-wobbled wave or waves in consideration of discordance among frequency characteristics of respective receiving-selection circuits at the receivers. If there are many kinds of the emergency control signals to be transmitted, a plurality of signal waves are employed instead of a single wave. Amplitude modulation or frequency modulation may be employed as the modulation system of this case. The emergency information will be broadcast after sending the above-mentioned emergency control signal.

On the other hand, the receiver side which includes a receiving set, such as a radio receiving set or a television set, is established in the standby condition for receiving the emergency control signal. In other words, the video pattern and/or the audio output of the receiving set are/is not seen or heard because of the inactive state of the video circuit and/or the audio circuit. The receiving set is designed so that if an emergency control signal is transmitted from the sending side, the receiving-and-selection circuit of the emergency control signal provided at a receiver receives and selects only the transmitted emergency control signal and then actuates the video circuit and/or the audio circuit. Accordingly, if a predetermined emergency control signal is received at a receiver, the video circuit and/or the audio circuit of the receiver is switched from the inactive state to the active state and the emergency information following after the emergency control signal can be received at the receiver eyes and/or ears.

In the foregoing system, it is essential in view of the nature of the emergency information, that emergency information transmitted is reliably transmitted to each of the receivers in 100 percent efficiency and that when there is no transmission of the emergency control signal, the receiving-and-selection circuit must not cause spurious triggering, when a signal similar to the emergency control signal (hereinafter called as false signal") is broadcast, the receiver must not be spuriously actuated. To meet these requirements, the receivingand-selection circuit of the emergency control signal in the conventional system is provided with a narrow band-pass filter or filters for an intermittent signal or signals, a complex gate circuit is provided with is designed in consideration of the duration, the intermittent period and the predetermined number of pulses of the intermittent signal waves. However, the more complex the gate circuit becomes to avoid the spurious triggering, the less the reliability of the detection operation in receiving a regular emergency control signal becomes.

To avoid spurious triggering, a further technique is proposed technique in which the emergency control signal is formed into a configuration which could not occur in actual broadcast programs. However, even though the configuration of the emergency control signal is established to avoid spurious triggering by existing music and speech broadcast, music having new rhythms and melodies may be composable infinitely. Accordingly, even if such requirement that the emergency control signal is formed into the configuration which could not occur in actual broadcast programs is satisfied, and even if such defects as complex formation and high price of the receiving-and-selection circuit are neglected, it is impossible that the emergency control signal is stably and reliably transmitted to each of the receivers so as to make it receive the emergency information only.

Moreover, since the noise in the transmission medium must be taken under consideration in actual systems, the abovementioned unstableness will further increase.

SUMMARY OF THE INVENTION without spurious triggering, switching the receiving condition of the receiving side from the standby state to the receivable state for a broadcast information signal which is transmitted after the broadcasting of the control signal.

Further object of this invention is to provide a broadcast system capable of, by transmission of a control signal without spurious triggering, controlling a device provided at the receiving side for receiving the control signal only.

The principle of this invention will be understood from the following detailed discussion taken in conjunction with the accompanying drawings, in which the same or equivalent parts are designated by the same reference numerals, characters, and symbols, and in which:

FIG. 1 shows time charts explanatory of an example of the control signal used in the system of this invention and false signals similar to the control signal;

FIG. 2 is a block diagram illustrating an example of the receiving side of the system of this invention;

FIG. 3 is a circuit diagram illustrating an example of a circuit used in the circuitry shown in FIG. 2 or the sending side of the system of this invention;

FIG. 4 is time charts explanatory of the operation of the circuit shown in FIG. 3;

FIG. 5 is a block diagram illustrating an example of the sending side of this invention;

FIG. 6 is a block diagram illustrating an example of the decision circuit used in the example shown in FIG. 5;

FIG. 7 and 8 show time charts explanatory of the operations of the decision circuit shown in FIG. 6;

FIG. 9 is a block diagram illustrating another example of the sending side of the system of this invention; and

FIG. 10 shows a time chart explanatory of the operation of the example shown in FIG. 9.

In the system of this invention, it is assumed that the control system is substantially a continuous wave w of a single frequency lasting a duration more than a time T as shown in FIG. 1. Accordingly, if this continuous wave is continuously received at a duration T, an audio circuit and/or a video circuit of the receiving side are/is actuated in response to the output of a selection circuit for the control signal. This is referred as normal triggering hereinafter. Accordingly, since a wave such as a w, having a duration less than the time T or a wave w having durations t and t each less than the time T and intermittent time 7 between the durations t and cannot perform the above-mentioned normal triggering.

In consideration of the above-mentioned format of the control signal, a receiving side is formed as shown in FIG. 2, by way of example. In this example, the demodulated output of a receiver REC is applied to a selection circuit for control signal which comprises a first circuit I and a second circuit II. The first circuit I comprises a narrow band-pass filter 3 passing therethrough only signals of frequencies included within a narrow frequency band whose center frequency is the frequency of the control signal, an amplifier 4 amplifying the output of the narrow passband filter 3, and a rectifier circuit 5 converting the AC output of the amplifier 4 to a DC signal. The second circuit II (i.e., control circuit) generates an output when the output of the first circuit I continues at a duration more than the time T to perform the above-mentioned normal triggering. This control circuit II may be designed as digital type or analogue type. FIG. 3 shows an example of the control circuit II formed into the analogue type.

With reference to FIG. 4, the operation of this example shown in FIG. 3 will be described. A DC signal v obtained from the circuit I shown in FIG. 2 is applied to the baseemitter path of a Transistor TR Accordingly, the potential of a point 7 (i.e., the collector of the Transistor TR,) against the ground varies as a voltage v,, so that a voltage across a capacitor C connected across a point 8 and the ground varies as a voltage v In this case if the time constant of a time delay circuit comprising a resistor R, and a capacitor C is determined so that the collector current i, of Transistor TR starts as shown in FIG. 4 when the duration of the control signal transv mitted reaches a predetermined time T, a contract of a relay 6 is closed in response to the start of the collector current i of the Transistor TR when the duration of the transmitted control signal reaches the predetermined time T.

The above operation is the normal operation in a case where a correct control signal is transmitted. However, if a false signal the duration of which is less than the predetermined time T while the frequency thereof is equal to the frequency of the correct control signal is applied to this circuit I, the charged voltage v is discharged through a path: the capacitor C, a resistor R,, and the collector-emitter path of the Transistor TR The discharging time mo of this path determined in accordance with a product of respective values of the resistor R, and the capacitor C is considerably smaller than the charging time constant for the capacitor C determined in accordance with a product of respective values of the resistor R, and the capacitor C. Accordingly, even if an intermittent space of the false signal is relatively short but longer than an allowable maximum intermediate time 1-,, which is determined to detect continuity of the control signal (e.g., the false signal) and substantially equal to the discharging time constant R C. The charged voltage across the capacitor C is completely discharged within the intermittent space while correct triggering is performed without errors when a correct control signal having a duration more than the time T is received.

In consideration of the condition of the selection circuit (i.e., circuits I and II) of the control signal provided at the receiving side of this broadcast system, it will be understood that if detection means is provided at the sending side of the broadcast system to detect a false signal before expiration of the time T and if elimination means is provided at the sending side of the broadcast system to eliminate the same frequency as the control signal from the broadcast program signal during a period 7a longer than the above-mentioned discharging time constant R,C (i.e., the allowable maximum intermittent time 1-, determined in accordance with the respective values of the resistor R and the capacitor C shown in FIG. 3, spurious triggerings can be completely eliminated from this broadcast system.

An example of the auxiliary circuitry comprising the detection means and the elimination means each provided at the sending side of this broadcast system will be described with reference to FIG. 5. In FIG. 5, the broadcast program signal different from the control signal is applied from a terminal 10 and passes through a switch circuit 11, a connection line 12, a switch circuit 13, and an amplifier 14, an OR-circuit and a connection line 16 to be applied to a modulator of a transmitter. If a false signal of a frequency substantially equal to the frequency of the control signal is included in the broadcast program signal, a detector 17 detects the false signal exceeding a threshold level and applied it to a next decision circuit 18 as a DC signal. This decision circuit 18 generates an output in consideration of the predetermined decision principle thereof. In other words, if the decision circuit 18 detects that the duration of the DC output of the detector 17 exceeds a predetermined time Ta less than the time T of the regular control signal, the decision circuit 18 generates a particular output signal to control the switch circuits 1 1 and 13 through the OR- circuit 19 and a control circuit 20. In response to this operation, the broadcast signal applied from the terminal 10 is switched to a narrow band-elimination filter 21 eliminating signals of frequencies approximate to the frequency of the control signal. This time (1a) of inserting the narrow band-,

elimination filter 21 is determined so as to be longer than the allowable maximum intermittent time 1-,, as mentioned above.

On the other hand, the control signal is applied from a terminal 22 and applied to the modulator of the transmitter through the OR-circuit l5 and the connection line 16. Accordingly, the control signal is never detected by the detector 17. If the control signal is applied to the modulator of the transmitter simultaneously with the broadcast program signal, it is desirable that signals of frequencies approximate to the frequency of the control signal are eliminated from the broadcast program signals by switching-in the narrow band-elimination filter 21 in the path of the broadcast program during the duration of the control signal.

The detector 17 may be designed similarly to the circuit I of the selection circuit provided at the receiving side of the broadcast system as shown in FIG. 2. However, it is necessary that the sensitivity of the detector 17 is higher than the selection circuit of the receiving side and that the frequency band of a narrow band-pass filter provided in the detection circuit 17 is wider than the frequency band of the filter 3 of the receiving side in consideration of deviations of the filters 3 of all receiving sets.

The decision circuit 18 may be similarly designed for the analogue circuitry shown in FIG. 3. However, digital circuitry is desirable for the decision circuit 18 in consideration to drop of the accuracy for measuring the duration of the control signal caused by deviation of values of elements of the timemeasuring circuit (i.e., the charging time constant and/or the discharging time constant of the capacitor C). An example of this decision circuit 18 of digital type will be described with reference to FIGS. 6, 7 and 8. In the example shown in FIG. 6, a timing pulse generator TP generates four pulse trains P P P and I whose respective pulses are timed with time slots designated in FIGS. 7 and 8 by numerals 0, I, 2 and 3 each corresponding to respective suffixes of the reference P P P: and P A clock circuit CK is a counter counting the number of pulses e applied from an OR-circuit OR,. The counting state of the counter CK is reset in response to each of pulses i applied from an OR-circuit 0R The clock circuit CK has a first output terminal 0 and a second output terminal 0 When the duration of the false signal reaches the predetermined time Ta, a pulse is generated from the first output terminal 0,. This operation is performed when the counter of the clock circuit CK counts a predetermined first number of pulses e. On the other hand, when a predetermined time Ta-l-ra starting from the start of the false signal expires, the second output terminal 0 generates an output pulse. This operation is performed when the counter of the clock circuit CK counts a predetermined number of pulses e larger than the first number of pulses e.

Decision operation of this decision circuit 18 will be described with reference to FIG. 7 in a case where the duration of a false signal is less than the predetermined time Ta. The false signal is detected by the detection circuit 17 and applied to an input terminal 31 as a DC signal a. This DC signal a is sampled at an AND-circuit AND, by the use of pulses of the pulse train P so that a pulse train b is obtained at the output of the AND-circuit AND,. This sampling is performed to give regular timing to the false signal occuring at random. A bistable circuit F,, such as a Flip-Flop circuit, is set by each pulse of the pulse train b and reset by each pulse of the pulse train P so that pulse trains c and f are obtained respectively at two output terminals reversely polarized to each other at the set state and reset state of the bistable circuit F The pulses train c is sampled at an AND-circuit AND, by the use of pulses of the pulse train P, so that a pulse train d is obtained at the output of the AND-circuit AND Tl-Iis pulse train :1 passes through an OR-circuit OR,. The number of the output pulses e of the OR-circuit OR, is counted by the counter provided in the clock circuit CK. On the other hand, pulses of the pulse train f is sampled at an AND-circuit AND, by the use of the pulse train P,, so that a pulse train g is obtained at the output of the AND-circuit AND Since any output pulse is not obtained at the output terminal 0, so that a bistable circuit F is not set, the output n of the bistable circuit F assumes continuously the state 1. Accordingly, pulses of the pulse train 3 passes through an AND-circuit AND The output pulses h of the AND-circuit AND, passes through the OR-circuit OR, and resets the counting state of the counter provided in the clock circuit CK for each pulse.

As mentioned above, if the duration of each of the false signals is less than the predetermined time Ta, any of output pulses are not generated from the clock circuit CK since the counting state of the counter provided in the clock circuit CK is reset to the initial state by the output pulses i of the OR-circuit 0R before the counter generates a carry pulse. Accordingly, the state of the output m of the bistable circuit F obtained at an output terminal 32 is not changed in this case so that the output of this decision circuit 18 does not control the control circuit 20.

With reference to FIG. 8, the operation of the decision circuit 18 in a case where a false signal having a duration longer than the predetermined time Ta occurs will be described. In this case, it is assumed that the scale of the counter provided in the clock circuit CK is equal to the number of periods of the pulses e (i.e., pulses P,) corresponding to the predetermined time Ta, a Ta second pulse is obtained at the output 0, of the clock circuit CK when the duration of the false signal reaches the predetermined time Ta. This pulse passes through an AN D-circuit AND, in timing with pulses of the pulse train P The output of the AND-circuit AND, sets the state of the bistable circuit F so that the state of the output m of the bistable circuit F is changed to the state 1. Accordingly, the band-elimination filter 21 is inserted in the path of the broadcast program in response to this change to the state 1 ofthe output m.

In this case, since pulses of the pulse train P, passes through an AND-circuit AND, opened in response to the above-mentioned change to the state 1 of the output m, the output pulses q of the AND-circuit AND, are applied to the clock circuit CK through the OR-circuit OR,. On the other hand, since the other output n of the bistable circuit F assumes the state 0 in response to the state I of the output m, the output pulses g of the AND-circuit AND, is checked at the AND-circuit AND Accordingly, the pulses e are further applied to the clock circuit CK even if the pulses d are terminated in response to the termination of the false signal. If the clock circuit CK is designed so as to generate a (Ta+'ra) second pulse when the number of periods of the pulses e reaches a number corresponding to a time (Ta-Pm) starting from the start of the false signal, the (Ta+'ra) second pulse is obtained at the output terminal 0 when the time Ta is over from the termination of the false signal. This (Ta-l-ra) second pulse passes through an AND-circuit AND, in timing with one of pulses of the pulse train P The output pulse k of the AND-circuit AND resets the bistable circuit F to the initial state and sets the state of a bistable circuit F In response to this change to the state l of the output m, the band-elimination filter 21 is inserted as shown by a rederence BEF in the path of the broadcast program. At the same time, the output s of state l of the bistable circuit F, passes through an AND-circuit AND, in timing with one pulse of the pulse train P,. The output pulse r of the AND-circuit AND passes through the OR-circuit OR and resets the counter provided in the clock circuit CK. On the other hand, the AND-circuit AND, is opened in response to the reset of the bistable circuit F The above-mentioned operations are repeated in response to each of the false signals each of which has a duration longer than the time Ta.

As mentioned above, the example of the decision circuit 18 shown in FIG. 6 comprises the clock circuit CK having the first output terminal 0, and the second output terminal 0,; the bistable circuit F first input means comprising the input terminal 31, the AND-gate AND,, the bistable circuit F,, the AND-gate AND, and the OR-circuit OR,; second input means comprising the AND-gate AND, and the OR-circuit OR,; first resetting means comprising the AND-gate AND,, the AND- gate AND, and the OR-circuit 0R second resetting means comprising the bistable circuit F;;, the AND-gate AND, and the OR-circuit CR and the timing pulse generator TP. The first input means converts a DC signal obtained from a false signal having the same frequency as the control signal to a pulse train d (i.e., e) timed with the pulse train P, generated from the timing pulse generator TP. The second input means generates the pulse train q (i.e., e) timed with the same pulse train P, by the use of the output of the bistable circuit F, during a duration where the false signal intermittes. The clock circuit CK is provided with a counter counting the number of pulses applied from the first and second input means. When a predetermined first number of pulses e whose number of periods corresponds to the predetermined time Ta less than the regular duration T of the control signal is counted by the counter, the counter provided in the clock circuit CK generates a Ta second pulse at the first output 0,. This operation is measuring of the duration of the false signal. Moreover, when a predetermined second number of pulses e whose number of periods corresponds to the predetermined time ra more than the minimum allowable intermittent time 1- of the false signal in the receiving side is counted by the counter after the completion of counting the first number of pulses e, the counter provided in the clock circuit CK generates a (Tamil) second pulse at the second output 0 The bistable circuit F is set by the use of the Ta second pulse and reset by the use of the (Ta-Ha) second pulse. If the duration of the false signal is less than the time Ta, the counting state of the counter provided in the clock circuit CK is reset to the initial state by the first resetting means. Moreover, the counting state of the counter is reset to the initial state after generation of the (Ta+-ra) second pulse by the second resetting means. In accordance with the above-mentioned construction, the example of the decision circuit 18 shown in FIG. 6 is possible to control the control circuit 20 so as to insert the band-elimination filter 21 in the path of the broadcast program by the use of the output m of the bistable circuit F Another example of the sending side of this invention will be described with reference to FIGS. 9 and 10. In this example, a band-elimination filter 44 is inserted in disregard of existence or nonexistence of the false signal in the path of the broadcast program in a predetermined periodical manner in which the filter 44 is inserted in the above-mentioned time a which is longer than the maximum allowable intermittent time 1-,, every period (Ta-Ha). The broadcast program is applied from a terminal 47, and the control signal is applied from a terminal 49. At the normal condition, the broadcast program passes through switches 42 and 43 and is applied to the transmitter 45 connected to an antenna 46. The control signal is also applied through a connection line 50 to the transmitter 45. The counter 40 counts pulses of a clock pulse train applied from a pulse generator (not shown) and generates a pulse at every period (-ra+Ta). A control circuit 41 controls the switches 42 and 43 in response to each of the output pulses of the counter 40 so that the filter 44 is switched in the path of the broadcast program in the time 1a every period (Ta+ra) as shown in FIG. 10. In FIG. 10, a state (I) shows periods in which the filter 44 is inserted and a state (II) shows periods in which the filter 44 is switched off. When the filter 44 is inserted in the path of the broadcast program, frequency'components substantially equal to the frequency of the control signal is eliminated from the broadcast program. Since the same frequency signal as the control signal cannot remain in excess of the time Ta in the broadcast program signal transmitted by the sending side shown in FIG. 9, spurious triggering in the receiving side is effectively eliminated.

in the above, it is assumed that the control signal is a continuous signal of single frequency. However, the control signal may be a combination of a plurality of continuous signals each having a single frequency. In this case, the above-mentioned protection technique against spurious triggering is applied to each of a plurality of continuous signals at the sending side of this broadcast system. As understood from the above, the frequency of the single continuous signal or the frequency of each of a plurality of continuous signals may be wobbled along a low-frequency signal in a narrow frequency deviation range in consideration of deviation of the center frequency of the filter 3 of the receiving side. In this case, the frequency band of the band elimination filter 21 or 44 is designed so as to cover the narrow deviation range.

What I claim is:

l. A broadcast system for transmitting a control signal from the sending side to the receiving side to trigger the receiving side by the control signal, comprising:

elimination means for eliminating the same frequency components as the control signal from a broadcast program signal being transmitted through a transmission medium of the broadcast system; detection means coupled to the path of the broadcast program signal to detect from the broadcast program signal a false signal having the same frequency as the control signal before the receiving side is spuriously triggered by the false signal, the detection means comprising selection means for selecting the same frequency signal as the control signal from the broadcast program signal, and measuring means for measuring the duration of the same frequency signal selected to generate a control output only when the continuous duration of the same frequency signal selected exceeds a first predetermined time less than the regular duration of the control signal; and

control means coupled to the elimination means and the output of the detection means to insert during only a predetermined second time said elimination means into the path of the broadcast program signal in response to the control output generated from the measuring means of the detection means.

2. A broadcast system according to claim 1, in which said measuring means comprises a capacitor, charging means for charging the capacitor from the start of the same frequency signal selected by said selection means, discharging means for discharging the capacitor from the termination of the same frequency signal selected by said selection means, and means for generating the control output when the charged voltage exceeds a reference threshold level.

3. A broadcast system according to claim 1, in which said measuring means comprises first input means for generating a first pulse train only the duration of the same frequency signal selected by the selection means, second input means for generating a second pulse train only the intermittent time of the same frequency signal selected by the selection means, a counter for counting the number of pulses of the first and second pulse trains from the start of the first pulse train to generate a first output when the counter counts a first predetermined number of pulses corresponding to the first predetermined time and to generate a second output when the counter counts after the first output a second predetermined number of pulses corresponding to second predetermined time,'first resetting means for resetting the counting state of the counter when the duration of the same frequency signal selected by the selection means is terminated before said generation of the first output of the counter, and second resetting means for resetting the counting state of the counter by the use of the second output of the counter, whereby the first and second outputs of the counter are applied to the control means to determine the second predetermined time so that this second predetermined time starts from the first control output and terminates at the second control output.

4. A broadcast system for transmitting a control signal from the sending side to the receiving side to trigger the receiving side by the control signal, comprising:

elimination means for eliminating the same frequency components as the control signal from a broadcast program signal being transmitted through a transmission medium of the broadcast system;

means for generating control pulses each having a duration more than a maximum allowable intermittent time for detecting continuity of the control signal, at the receiving side said control pulses being generated every period less than the regular duration of the control signal; and control means coupled to the elimination means and the output of the just preceding means to insert during only a predetermined time corresponding to the duration of each of the control pulses said elimination means into the path of the broadcast program signal in response to each of the control pulses.

t l i 

1. A broadcast system for transmitting a control signal from the sending side to the receiving side to trigger the receiving side by the control signal, comprising: elimination means for eliminating the same frequency components as the control signal from a broadcast program signal being transmitted through a transmission medium of the broadcast system; detection means coupled to the path of the broadcast program signal to detect from the broadcast program signal a false signal having the same frequency as the control signal before the receiving side is spuriously triggered by the false signal, the detection means comprising selection means for selecting the same frequency signal as the control signal from the broadcast program signal, and measuring means for measuring the duration of the same frequency signal selected to generate a control output only when the continuous duration of the same frequency signal selected exceeds a first predetermined time less than the regular duration of the control signal; and control means coupled to the elimination means and the output of the detection means to insert during only a predetermined second time said elimination means into the path of the broadcast program signal in response to the control output generated from the measuring means of the detection means.
 2. A broadcast system according to claim 1, in which said measuring means comprises a capacitor, charging means for charging the capacitor from the start of the same frequency signal selected by said selection means, discharging means for discharging the capacitor from the termination of the same frequency signal selected by said selection means, and means for generating the control output when the charged voltage exceeds a reference threshold level.
 3. A broadcast system according to claim 1, in which said measuring means comprises first input means for generating a first pulse train only the duration of the same frequency signal selected by the selection means, second input means for generating a second pulse train only the intermittent time of the same frequeNcy signal selected by the selection means, a counter for counting the number of pulses of the first and second pulse trains from the start of the first pulse train to generate a first output when the counter counts a first predetermined number of pulses corresponding to the first predetermined time and to generate a second output when the counter counts after the first output a second predetermined number of pulses corresponding to second predetermined time, first resetting means for resetting the counting state of the counter when the duration of the same frequency signal selected by the selection means is terminated before said generation of the first output of the counter, and second resetting means for resetting the counting state of the counter by the use of the second output of the counter, whereby the first and second outputs of the counter are applied to the control means to determine the second predetermined time so that this second predetermined time starts from the first control output and terminates at the second control output.
 4. A broadcast system for transmitting a control signal from the sending side to the receiving side to trigger the receiving side by the control signal, comprising: elimination means for eliminating the same frequency components as the control signal from a broadcast program signal being transmitted through a transmission medium of the broadcast system; means for generating control pulses each having a duration more than a maximum allowable intermittent time for detecting continuity of the control signal, at the receiving side said control pulses being generated every period less than the regular duration of the control signal; and control means coupled to the elimination means and the output of the just preceding means to insert during only a predetermined time corresponding to the duration of each of the control pulses said elimination means into the path of the broadcast program signal in response to each of the control pulses. 